Yogapedia explains Sunyata

Sunyata is a commonly used concept in Buddhism that can be interpreted in various ways depending on the context. It can refer to a meditative experience or state. It is also said that the realization of sunyata is the key to attaining enlightenment and true wisdom.

Reaching this state means the yogi has realized that the five skandhas (the elements that make up the human being: form, perception, sensation, consciousness and mental formation) are impermanent and not related to any sort of inner Self. In this context, sunyata refers to the fact that we are essentially empty of the individual Self that we tend to think of as being “us.” This is a crucial concept in Buddhism.